Oxygen and aluminum-magnesium isotope systematics of the metasomatically altered coarse-grained igneous calcium-aluminum-rich from CK3.7 − 3.8 carbonaceous chondrites

Alexander N. Krot, Kazuhide Nagashima, Tasha L. Dunn, Chi Ma, Michail I. Petaev

Geochimica et Cosmochimica Acta
In Press, Journal Pre-proof, Available online 23 January 2025

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“We report on oxygen and aluminum-magnesium isotope systematics of Compact Type A (CTA), Type B (B), and Forsterite-bearing Type B (FoB) Ca,Al-rich inclusions (CAIs) from the Northwest Africa (NWA) 5343 (CK3.7) and NWA 4964 (CK3.8) chondrites that experienced metasomatic alteration in the presence of aqueous solution that resulted in replacement of primary melilite, AlTi-diopside, grossmanite, anorthite, and perovskite by secondary minerals. The primary minerals have excesses of radiogenic 26Mg (26Mg) that correlate with 27Al/24Mg ratio; the only exception is melilite in the CTA CAI. The calculated internal Al-Mg isochrons in the CTA (excluding melilite), Type B, and FoB CAIs correspond to the initial 26Al/27Al ratios [(26Al/27Al)0] of (5.09 ± 0.58) × 10−5, (2.58 ± 3.2) × 10−5, and (5.05 ± 0.66) × 10−5, respectively. The gehlenitic melilite (Åk<1) in the CTA CAI has resolvable 26Mg but very high 27Al/24Mg (up to ∼ 660) and does not belong to the internal isochron defined by hibonite, spinel, and grossmanite. The high 27Al/24Mg in melilite containing submicron inclusions of grossular is due to redistribution of Mg between these minerals during thermal metamorphism. Hibonite, spinel, forsterite, rhönite/louisfuchsite, and a grossmanite inclusion in spinel have 16O-rich compositions (Δ17O ∼  − 23 ± 2 ‰), whereas melilite, anorthite, and perovskite are 16O-poor (Δ17O ∼  − 3 ± 2 ‰). Grossmanite and AlTi-diopside are 16O-depleted to various degrees: Δ17O ranges from ∼  − 24 to ∼  − 3 ‰; the degree of 16O-depletion correlates with titanium content in pyroxene. On a three-isotope oxygen diagram secondary grossular, FeAl-diopside, FeMg-olivine, and plagioclase plot along mass-dependent fractionation line with Δ17O of ∼  − 3.7 ± 1.9 ‰ that corresponds to Δ 17O of metasomatic fluid in the host meteorites. This value is indistinguishable from Δ 17O of metasomatic fluid that resulted in alteration of Allende (CV > 3.6) CAIs.
Coarse-grained igneous CAIs in CKs and CVs have similar size distribution, textures and primary mineralogy, formed in a gas of approximately solar O-isotope composition (Δ 17O ∼  − 24 ± 2 ‰) and had the canonical (26Al/27Al)0, suggesting they belong to the same generation of refractory inclusions, further supporting genetic relationship between CVs and CKs. Oxygen-isotope heterogeneity in CV > 3.6 and CK3.7 − 3.8 CAIs resulted from postcrystallization O-isotope exchange with 16O-depleted metasomatic fluid (Δ 17O ∼  − 3.7 ± 1.9 ‰) on their parent asteroid(s). This exchange preferentially affected melilite, anorthite, perovskite, and AlTi-pyroxenes, whereas hibonite, spinel, rhönite/louisfuchsite, and forsterite retained their original 16O-rich compositions established during igneous crystallization in a gas of approximately solar composition. Metasomatic alteration and thermal metamorphism of CAIs from CK3.7 − 3.8 and CV > 3.6 chondrites disturbed their Al-Mg isotope systematics to various degrees.”